Inkjet printing processes fall into two main types: continuous processes and drop-on-demand (DOD) processes. Continuous processes use electrically conductive inks to produce a stream of electrically-charged ink drops that are deflected by an electric field to an appropriate location on a substrate. In DOD processes, individual drops of ink are expelled from the nozzle of a printhead either by vibration of a piezoelectric actuator (in piezoelectric inkjet printing) or by heating the ink to form a vapourised gas bubble (in thermal inkjet printing, also known as bubblejet printing).
Inks for ink jet printing are required to have viscosity values that will enable effective ink flow through the fine nozzles of the printhead without causing clogging of the same. Considerable effort has therefore been extended to the successful development of cyan, magenta, yellow and black (CMYK) inks for full colour printing acceptable for use in inkjet printers; however providing an acceptable white inkjet ink presents particular problems.
White inks used to form the undercoat or base layer on a substrate need to have good hiding power in printed form. Conventional white inkjet ink compositions typically rely on particulates (pigments) to achieve the required opacity desirable of a film for use as base layer coatings.
An inkjet printing ink composition containing such particulates, however, presents special problems. For example, the particulates must be ground to acceptable sizes to prevent the particulates from plugging the small printhead orifices. A small particulate size is further necessary to ensure that uniform ink droplets are reliably formed at the printhead nozzle plate.
Titanium dioxide (TiO2) is one such pigment which has been widely applied to the development of white inkjet ink formulations. However, the dispersions of this pigment are hampered by the high specific gravity of the TiO2 rutile (4.2) and anatase (3.9) polymorphs, which cause the pigment to hard-settle when the ink is allowed to stand for an extended period of time. This lack of dispersion stability is partly circumvented by employing inkjet printers with integrated ink recirculation systems, enabling the ink to be agitated periodically to prevent hard settling of the pigment. However, TiO2 based white inks for inkjet printers reliant on the use of ink cartridges are totally impracticable.
The use of TiO2 as an opacifying agent in white inkjet inks is further precluded on the grounds of the extreme hardness of this ceramic, which accelerates wear to the nozzle portions of an inkjet printhead. Other less hardwearing inorganic white colourants that have been applied to inks include zinc oxide, barium sulfate, and certain clays, while a number of organic colourants such as the resin fillers based on acrylic, styrene, melamine, or the like, have also been employed.
JP 63254176, JP2000-103995, and U.S. Pat. No. 4,880,465 all disclose white ink formulations comprising hollow polymer microparticles as the white colourant in the ink. Hollow polymer microparticles have a specific gravity close to 1 and are therefore less prone to settling, thereby providing a solution to the problem of ink clogging caused by the settling of pigment.
The prior art also discloses radiation-curable white inkjet ink compositions that include monomers that are curable in response to appropriate stimulus (ultraviolet (UV), infra red (IR), microwave or heat). However, those known TiO2 based white inks are hindered by the ability of the pigment to reflect UV light, thus preventing the ink film from curing fully (see Japanese Laid-Open Patent Application No. H9-183929).
A number of prior art documents disclose resin-based inkjet inks comprising neither pigment nor dye, which are capable of producing opaque films through evaporation of the liquid vehicle. The resulting films comprise a multiplicity of microvoids that scatter incident light thereby producing an opaque image.
For example, U.S. Pat. No. 4,389,503 concerns non-pigmented inkjet inks comprising film-forming cellulose ester, resin binding agent and a solvent blend. The solvent blend comprises a volatile organic solvent component in which the film former and resin are soluble, e.g. methanol, acetone etc. and a less volatile (non-solvent) component (preferably water) in which the film former is essentially insoluble. The film former is initially fully dissolved in the solvent blend. On application, the volatile component evaporates rapidly (in 2 to 3 seconds), initially leaving behind the non-solvent. On subsequent evaporation of the non-solvent microvoids are produced, providing the opaque effect. The compositions thus use differential evaporation rates to produce voids. Such a composition relies on the fact that after its application to a substrate the solvent/non-solvent ratio of the solvent blend shifts so that the deposited ink becomes rich in non-solvent and deficient in solvent. In effect, phase-inversion occurs, causing the formation of a film having a plurality of microvoids. One of the problems associated with this ink is that the proper formation of the microvoids is critically dependent upon the conditions under which the ink is deposited. Further, because a delicate balance between the solvent/non-solvent ratio must be retained, the composition must be stored and used only under tightly controlled conditions: otherwise the ink is not able to maintain the ink solvent balance necessary to preserve a stable image and excessive variation in the quality of the opaque image results.
U.S. Pat. No. 5,710,195 and U.S. Pat. No. 5,674,923 similarly concern inkjet inks for producing opaque, non-pigmented film, using a solvent blend to obtain differential evaporation and hence production of voids.
U.S. Pat. No. 4,207,577 concerns inkjet inks for producing opaque images comprising resins/polymers, especially celluloses (for adhering to a substrate), solvent for the resin comprising a mixture of an aliphatic monovalent alcohol e.g. methanol etc. and a keytone/aldehyde/ether/ester/hydrocarbon/glycol/glycol ether/lactone etc., e.g. a mixture of methanol and methyl ethyl ketone. An opaque image is obtained by applying moisture to a printed image or by adjusting the temperature to the dew point.
U.S. Pat. No. 4,258,367 concerns inkjet inks including a light sensitive diazonium compound that responds to UV to produce an opaque image.
JP 2005-298757 concerns UV-curable inkjet inks that produce non-pigmented white opaque images. The inks use low boiling point solvents (having a boiling point in the range 40 to 110° C.) that evaporate to produce pores.
However, these inks have been formulated using low boiling solvent vehicles to enable fast evaporation of the solvent. The use of low boiling solvents generally precludes the ability to form and maintain a stable, high quality opaque image, due to excessive variation in the quality of the final film.
U.S. Pat. No. 3,823,027 concerns relatively thick opaque coatings, not needing pigment, that are produced from compositions comprising curable actinic light-sensitive monomers or pre-polymers, e.g. acrylic resins, and 10 to 70% solvent having a boiling point of at least 115° F. (46° C.) and a flash point of at least 50° F. (10° C.). The extent of solubility of the solvent with the curable materials is critical for the generation of voids, and hence opacity, during curing on exposure to actinic radiation. The coatings are intended to be applied by roll coating, spraying or dip coating.